The present invention relates to nitrided mixed oxide catalysts and a process for the production of ethylenically unsaturated carboxylic acids or esters, particularly α, β unsaturated carboxylic acids or esters, more particularly (alk)acrylic acids or alkyl (alk)acrylates such as (meth)acrylic acid or alkyl (meth)acrylates by the condensation of carboxylic acids or esters with a methylene or ethylene source, such as formaldehyde or a suitable source thereof in the presence of nitrided mixed oxide catalysts. In particular, but not exclusively, the invention relates to a process for the production of (meth) acrylic acid or alkyl esters thereof, for example, methyl methacrylate, by the condensation of propionic acid or alkyl esters thereof with formaldehyde or a source thereof in the presence of such nitrided mixed oxide catalysts.